A variety of particulate slagging compositions, also referred to as "mold powders", "slags", or "fluxes", have been proposed for the continuous casting of steels, a fairly recent development in steel mill practice. Such materials protect the molten metal from air oxidation while usually fluxing or solubilizing and thereby removing some oxide impurities present in the steel melt. Additionally, lubrication of the mold often can be enhanced by the use of such materials. Typically, the material is fed or poured on the top surface of the molten metal. Occasionally this top is referred to as the meniscus.
In the trade the terms "flux", "slag", or "mold powders" often have been used interchangeably for fritted or preponderantly fritted material to be used in continuous casting service. For convenience, a particulate slagging composition will be defined as encompassing all types of materials used to protect and lubricate the steel during continuous casting. A "vitrifaction" will be defined as a totally vitrified (fritted) material or mixture of fritted material for the instant purpose. A "flux" will be a vitrifaction to which there is added non-vitrified material in small proportion, that is, less than about 30% of the total flux. Separate from both flux and vitrifaction, are "mold powders" which shall be defined as essentially raw materials which have not been vitrified to any appreciable extent. Typically, the instant particulate slagging compositions without carbon are made by comminuting the components and/or vitrified components, then blending if necessary. The exemplified vitreous fluxes shown in U.S. Pat. No. 3,926,246 and U.S. Pat. No. 4,092,159 feature a portion of the fluorine-providing material mixed unfritted with the balance of the glass formers which are fritted. This is done to minimize smelter attack during making of the fritted portion of the particulate slagging composition. Ordinarily from 1-10% and preferably 1-5% by weight of powdered graphite is added to make the final continuous casting particulate slagging composition. Such graphite is for the purpose of minimizing heat loss from the surface of the molten metal.
Steels now continuously cast in production operations include various aluminum-killed steel, silicon-killed steel, and austenitic stainless steel. The problems involved in formulating a particulate slagging composition for use in continuous casting steel have been addressed in previous industry literature. Specifically, U.S. Pat. Nos. 3,649,249, 3,704,744, and 3,899,324 show some of the attempts by others to maximize the performance of the particulate slagging compositions used.
Recongnition has also been given in the industry literature to a problem concerning the absorption of alumina into the melted slagging composition during continuous casting. The alumina comes from the steel being cast. The problem is the most severe when aluminum-killed steel is being cast. Of course, aluminum-killed steel is the predominant type of steel produced by continuous casters. The absorption of alumina into the particulate slagging composition leads to an eventual deterioration in performance characteristics. As the continuous casting run of steel is extended in time, more and more alumina is absorbed into the molten particulate slagging composition. After a certain optimum casting run length, the molten particulate slagging composition's performance so deteriorates that the caster's steel output must be slowed down because the molten composition cannot transfer heat away from the forming solid steel shell fast enough to thicken the shell sufficiently. Also, the surface of the steel being cast shows more and more inclusions because the molten slagging composition cannot absorb impurities, primarily alumina, from the molten steel fast enough. The certain optimum casting run length differs for each individual caster and type of steel being cast. The amount of protection the molten steel receives from the air while on its way to and in going through the caster influences the amount of alumina created and later evolved during casting. In some casters the optimum casting run length can be as short as 45 minutes. This length is even shorter than the time required for a single heat of steel to be cast. In some casters, with much better protection, the optimum casting run length can be up to 8 or more hours, representing casting of several heats of steel without interruption.
The performance characteristics of the molten particulate slagging composition can deteriorate to such an extent that unacceptable surfaces on the steel being cast result. Also, with the evolution of more and more alumina into the slag, the viscosity of the molten composition can rise to such a high value that necessary lubrication of the mold is no longer provided. The rise in viscosity can hinder the liquid slagging composition's movement into the space between the mold wall and the forming solid steel shell. When the gap is unlubricated due to the absence of liquid slag, the steel shell can seize on the mold wall and the resulting danger of a breakout becomes unacceptable. Finally, the heat transfer value can lessen to such an extent that not a thick enough solid steel shell is created in the mold and the chances for a breakout through a smaller hole also become unacceptable. When any of these three things, or a combination of things, happens the caster must either be shut down immediately or the casting run interrupted. These shutdowns or interruptions happen in spite of the fact that the molten pool of particulate slagging composition covering the molten steel receives continuous additions of the unmelted particulate slagging composition. Thus, the problem of alumina absorption is more than just the problem of adding more mold powder, which of course is itself expensive. The alumina absorption problem, in fact, leads to shorter, inefficient, costly casting runs on the continuous caster.
Previous attempts to deal with this problem have focused on the so-called "V" ratio. The "V" ratio is generally defined as the lime to silica ratio. Koenig and Hofmanner in U.S. Pat. No. 3,788,840 require a lime-silica ratio in the flux powder to be in the range of 0.7-1.0. This arrangement is achieved by the addition of quartz powder. Koenig and Hofmanner in their '840 patent also require the aluminum oxide content of the powder to be in the range of 2-12% by weight. While helping to improve the performance characteristics of their flux powder on a continuous casting run, the flux powder cannot withstand the addition of great amounts of alumina experienced in an extended run and allow optimum casting to continue. An advantage of the instant particulate slagging composition is the greater ability to absorb more alumina, thus extending the length of the optimum continuous casting run possible.